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Journal of Clinical Microbiology, March 2006, p. 923-927, Vol. 44, No. 3
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.3.923-927.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Characteristics of Streptococcus pseudopneumoniae Isolated from Purulent Sputum Samples

Elaine R. Keith,¹ Roslyn G. Podmore,¹ Trevor P. Anderson,¹ and David R. Murdoch^1,2*

Microbiology Unit, Canterbury Health Laboratories, Christchurch, New Zealand,¹ Department of Pathology, Christchurch School of Medicine and Health Sciences, Christchurch, New Zealand²

Received 6 October 2005/ Returned for modification 20 November 2005/ Accepted 19 December 2005

Top Abstract Introduction Materials and Methods Results Discussion References

 
Streptococcus pseudopneumoniae is a recently described streptococcus that is phenotypically and genetically distinct from Streptococcus pneumoniae and other viridans streptococci. Key characteristics of S. pseudopneumoniae are the absence of a pneumococcal capsule, insolubility in bile, resistance or indeterminate susceptibility to optochin when incubated in 5% CO₂ but susceptibility to optochin when incubated in ambient air, and a positive reaction with the AccuProbe DNA probe hybridization test. The clinical importance of this bacterium is currently unknown. We report the characteristics and associated clinical data of 35 strains of S. pseudopneumoniae isolated from sputum samples from 33 patients. All isolates produced a positive result with the NOW S. pneumoniae antigen test (Binax, Inc.). No isolate was resistant to penicillin, but 60% were resistant to erythromycin and 77% were resistant to tetracycline. All patients had lower respiratory tract symptoms, 79% had chronic obstructive pulmonary disease (COPD), and 33% had chest radiographic infiltrates. Compared with matched control patients who had Streptococcus pneumoniae isolated from sputum, patients with S. pseudopneumoniae infection were more likely to have a history of COPD (odds ratio [OR], 5.0; 95% confidence interval [CI], 1.67 to 20.11) or exacerbation of COPD (OR, 6.5; 95% CI, 2.61 to 16.20). Further research is needed to better characterize the epidemiology of S. pseudopneumoniae colonization and the role of S. pseudopneumoniae in COPD and other diseases.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Streptococcus pseudopneumoniae is a recently described streptococcus that is phenotypically and genetically distinct from Streptococcus pneumoniae and other viridans streptococci (1, 5). DNA-DNA homology studies suggest that this species is a member of the Streptococcus mitis-Streptococcus oralis group (1), and it is likely that this species is similar to other strains previously described by several investigators as atypical pneumococci (4, 9, 12). S. pseudopneumoniae can be differentiated from S. pneumoniae and S. mitis by the absence of a pneumococcal capsule, demonstration of insolubility in bile, resistance or indeterminate susceptibility to optochin when incubated in 5% CO₂ but susceptibility to optochin when incubated in ambient air, and a positive reaction with a commercial DNA probe hybridization test (AccuProbe Streptococcus pneumoniae culture identification test; Gen-Probe, San Diego, CA).

Although the first-described isolates of S. pseudopneumoniae came from lower respiratory tract samples (1), the pathogenic potential and clinical importance of this bacterium are still undetermined. We report the characteristics and associated clinical data of 35 strains of S. pseudopneumoniae isolated from sputum samples.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Isolates. Since May 2001, we have been collecting consecutive alpha-hemolytic streptococcal strains isolated from sputum samples sent to our diagnostic laboratory. Only strains isolated from good-quality samples (>25 leukocytes and 10 squamous epithelial cells/x100 field) showing a Gram stain and culture predominance were archived. Isolates were identified as S. pseudopneumoniae on the basis of tests for pneumococcal capsule, bile solubility, optochin susceptibility, and AccuProbe DNA hybridization.

Bile solubility test. 0.5 ml of 2% deoxycholate was added to 0.5-ml suspensions of each isolate prepared in phosphate-buffered saline (PBS) and incubated at 35°C for 2 h. A positive test was indicated by visible clearing of the suspension.

Optochin susceptibility test. Sheep blood agar plates were inoculated with colonies from cultures grown overnight, and a 5-µg optochin disk was placed in the center of each inoculum. Each isolate was then incubated for 18 to 24 h at 35°C in both 5% CO₂ and ambient air environments. Optochin susceptibility was defined as a zone of inhibition of 14 mm.

DNA probe hybridization test. The AccuProbe Streptococcus pneumoniae culture identification test (Gen-Probe, San Diego, CA) was performed according to the manufacturer's instructions.

Detection of pneumococcal capsule. The Quellung test was used to detect the presence of a pneumococcal capsule. A light suspension of bacteria in PBS was air dried on a slide to which 5 µl polyvalent pneumococcal antisera (Omni Serum, Statens Serum Institut, Copenhagen, Denmark) diluted 1:4 in PBS and a small drop of methylene blue were added. A positive test was indicated by the presence of a sharply demarcated capsule when observed under oil immersion microscopy.

Rapid ID32 Strep identification system. The Rapid ID32 Strep identification system (bioMérieux, France) test was performed according to the manufacturer's instructions.

Pneumolysin gene (ply) PCR. The presence of the ply gene in extracted DNA from isolates was determined by PCR as previously described (7).

NOW S. pneumoniae immunochromatographic antigen test. The NOW S. pneumoniae immunochromatographic antigen test (Binax, Portland, ME) detects the C-polysaccharide cell wall antigen common to all S. pneumoniae strains. A single colony was touched by a swab, which was then placed into the test device. The test was then performed according to the manufacturer's instructions, with 6 drops of buffer solution added. A positive test result was indicated by the detection of both sample and control lines.

Antimicrobial susceptibility testing. Antimicrobial susceptibilities were determined by disk diffusion according to Clinical and Laboratory Standards Institute (CLSI) guidelines (2). Penicillin MICs were determined by the Etest (AB BIODISK, Solna, Sweden).

Clinical data. Demographic, clinical, and laboratory characteristics of all patients who had S. pseudopneumoniae isolated from sputum were obtained from a review of clinical records. For comparison, two control patients, matched for age and sex, were obtained for each case. These controls were identified from the same streptococcal database by selecting the next two patients chronologically after the case who were of the same gender and within 5 years of age and who did not have S. pseudopneumoniae isolated from sputum. The same data as for the cases were collected from the control patients. Mantel-Haenszel matched odds ratios were calculated for comparisons between cases and controls.

The study was approved by the Canterbury Ethics Committee.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Of 805 consecutive isolates collected between May 2001 and May 2004, 35 isolates (4%) from 33 patients were identified as S. pseudopneumoniae. All isolates lacked pneumococcal capsules, were bile insoluble, were resistant or had indeterminate susceptibility to optochin when incubated in CO₂ but were susceptible to optochin when incubated in an ambient environment, and had positive reactions with the AccuProbe DNA probe hybridization test. In addition, the colonial morphology of all isolates was similar to that of a type strain of S. pseudopneumoniae (NZRM4311; ATCC BAA-960). S. pseudopneumoniae colonies grown on sheep blood agar are typically small (up to 1 mm in diameter after 24 h of incubation), smooth, shiny, and domed, with entire edges. Occasional colonies have depressed centers, causing them to appear as a smaller version of the draftsmen colonies of S. pneumoniae. Other phenotypic and genotypic test results are shown in Table 1. All S. pseudopneumoniae isolates were collected before the first description of this species and had been originally identified as either alpha-hemolytic streptococci (33 isolates) or S. pneumoniae (2 isolates). Figure 1 shows a typical sputum Gram stain smear from a sample that grew S. pseudopneumoniae as the sole pathogen and illustrates the similar appearance to S. pneumoniae.

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TABLE 1. Phenotypic and genotypic test results for strains of S. pseudopneumoniae

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FIG. 1. Typical Gram stain appearance of a sputum smear that grew S. pseudopneumoniae as the sole pathogen.

Of the 35 S. pseudopneumoniae isolates, none was resistant to penicillin or vancomycin, 21 (60%) were resistant to erythromycin, 27 (77%) were resistant to tetracycline, and 4 (11%) were resistant to cotrimoxazole. The penicillin MICs ranged from <0.016 to 1.5 µg/ml (MIC at which 50% of the isolates are inhibited, 0.016 µg/ml).

Clinical, laboratory, and treatment data from the 33 patients with positive sputum cultures for S. pseudopneumoniae are presented in Table 2. One patient had S. pseudopneumoniae isolated on three separate occasions over 17 months; the three isolates had identical phenotypic and genotypic characteristics, apart from a slightly different Rapid ID32 profile (Table 1), and only clinical data related to the first isolation were analyzed. The median age of the patients was 68 years (range, 15 to 89), 19 patients (58%) were male, and 30 patients (91%) were inpatients. All patients had lower respiratory tract symptoms with cough, and exacerbation of chronic obstructive pulmonary disease (COPD) was the primary reason for presentation in 17 (52%) cases. Spirometry data were available for 17 of the 26 cases with a history of COPD: 4 were of severity II, 5 were of severity III, and 8 were of severity IV according to the GOLD criteria (6). For those patients who had data available, 9/31 (29%) had peripheral leukocyte counts of >11 x 10⁹ cells/liter, and only 3/30 (10%) had a temperature of >37.5°C on presentation. In each of the 10 patients with mixed infections, S. pseudopneumoniae was clearly the predominant organism isolated. There were no observed differences between patients with mixed infections and other patients (data not shown). All the control patients had positive sputum cultures for S. pneumoniae, and 20 patients (30%) had positive sputum cultures for S. pneumoniae mixed with other respiratory pathogens (mainly Haemophilus influenzae or Moraxella catarrhalis). Table 3 compares some clinical features of cases and controls. A history of COPD and exacerbation of COPD as a presenting feature were both significantly more common among cases than among controls.

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TABLE 2. Characteristics of patients with S. pseudopneumoniae isolated from sputum

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TABLE 3. Clinical features of patients who had S. pseudopneumoniae cultured from sputum compared with control patients who had S. pneumoniae cultured from sputum

Top Abstract Introduction Materials and Methods Results Discussion References

 
Our findings indicate that S. pseudopneumoniae can be isolated from a small proportion of sputum samples and that these strains had been previously identified as either viridans streptococci contaminants or as unusual pneumococci. Although sharing many features in common with S. pneumoniae and S. mitis, S. pseudopneumoniae can be readily identified in a clinical laboratory by the combination of some simple phenotypic tests, particularly tests for optochin susceptibility and bile solubility. Interestingly, S. pseudopneumoniae, like S. mitis (8), produces a positive result with the NOW S. pneumoniae antigen test, indicating that the antigen is shared between the species. Unlike the findings previously reported by Arbique et al. (1), who detected the pneumolysin gene in all of their S. pseudopneumoniae strains, five of our isolates tested negative for this gene. In addition, we documented a high rate of resistance to erythromycin and tetracycline among the S. pseudopneumoniae isolates. Recently, a single strain of S. pseudopneumoniae was found to be resistant to erythromycin and tetracycline due to the presence of mef(E) and tet(O) genes, respectively (3).

It can be difficult to determine whether a microorganism is a respiratory pathogen or whether it is simply a respiratory tract colonizer. This is especially so in the setting of COPD, where lower respiratory tract colonization with potential pathogens may occur even during times of clinical stability (10). In the present study, all strains of S. pseudopneumoniae were isolated as the predominant or only microorganism from good-quality purulent sputum samples obtained from patients with lower respiratory tract symptoms. Furthermore, in all cases, the sputum smear Gram stain results indicated the presence of Streptococcus species as the predominant bacteria. These findings are difficult to ignore and provide supporting evidence of a potential pathogenic role of S. pseudopneumoniae. In addition, approximately one-third of these patients had chest radiographic infiltrates, and one-third had peripheral leukocytosis. However, all these findings do not constitute definitive evidence of the pathogenic potential of S. pseudopneumoniae, and more data are required to support this notion. It is interesting that about one-quarter of patients with S. pseudopneumoniae in sputum were not treated with antibiotics and that all but one of these patients clinically improved.

Our preliminary data indicate that the isolation of S. pseudopneumoniae from sputum was associated with both a history of COPD and exacerbation of COPD. Over three-quarters of cases had documented COPD, and for at least half of all cases, exacerbation of COPD was the primary reason for seeking medical attention. Due to the large proportion of cases with a history of COPD, we performed a case control study to determine whether this result was a true association or whether it just reflected the background rate of COPD among patients who submitted sputum samples to our tertiary hospital laboratory. The findings of this study indicate that COPD was significantly more common among patients with S. pseudopneumoniae isolated from sputum than among the control group of patients who did not.

This study has several limitations, and we must stress the preliminary nature of our findings. We relied on information retrospectively retrieved from clinical records. Some information from the time of sputum collection was sparse, limiting the ability to obtain a detailed clinical picture for some patients. We may have underestimated the number of patients with COPD and other respiratory diseases owing to missing information. To better characterize the potential role of S. pseudopneumoniae in the exacerbation of COPD, it will be essential to examine COPD patients with exacerbations and compare them to those who do not with regard to isolation of this organism. Ideally, it would be important to monitor a cohort of patients with COPD and examine them during both exacerbations and times of clinical stability. The appearance of S. pseudopneumoniae or a new strain of S. pseudopneumoniae during exacerbations would support the widely accepted hypothesis that acquisition of a new strain of bacteria plays a causative role in exacerbation of COPD (11). Further work also needs to focus on the natural habitat of S. pseudopneumoniae, the epidemiology of S. pseudopneumoniae colonization, and the role of S. pseudopneumoniae in infections outside the setting of COPD, including pneumonia.

S. pseudopneumoniae isolates have now been isolated from North America and New Zealand, and it is likely that some of the so-called atypical pneumococci reported from Europe (4, 9, 12) are the same species. Increased awareness of this species will help to better determine its prevalence and clinical importance. Preliminary data from this study should prompt further research to characterize the role of S. pseudopneumoniae in COPD.

 
We thank staff of the Microbiology Unit, Canterbury Health Laboratories, for their support throughout the project; Alvin Chua for performing the pneumolysin PCR assays; and the New Zealand Reference Culture Collection, Medical Section, Institute of Environmental Science and Research, Porirua, New Zealand, for providing the type strain.

 
* Corresponding author. Mailing address: Microbiology Unit, Canterbury Health Laboratories, P.O. Box 151, Christchurch, New Zealand. Phone: 64 3 364 1530. Fax: 64 3 364 0238. E-mail: david.murdoch{at}cdhb.govt.nz.

Top Abstract Introduction Materials and Methods Results Discussion References

 

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Journal of Clinical Microbiology, March 2006, p. 923-927, Vol. 44, No. 3
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.3.923-927.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Keith, E. R. Articles by Murdoch, D. R. Search for Related Content PubMed PubMed Citation Articles by Keith, E. R. Articles by Murdoch, D. R.